With the increasing usage of mobile telecommunications stations, such as mobile telephone terminals, by users in various situations and in various places, demands for additional functionality of the mobile terminals arise. Such a demand is e.g. the possibility to be able to place and receive telephone calls to or from other persons without being forced to use one or both hands for operating the mobile terminal. A well known solution to the problem of operating the mobile terminal station without using the hands is to use a hands-free unit which is connected to the telephone by means of contacts on the housing of the mobile terminal and the hands-free unit. The hands-free unit may be in form of a microphone and an earphone coupled to the mobile terminal by means of a thin flexible cable, or in form of a docking station in a car connecting the mobile terminal to a microphone and a loudspeaker when the telephone is placed in the docking station. In addition to the transducer elements the hands-free unit preferably comprises suitable electronic circuitry such as amplifiers and interface circuits. Moreover, the docking station is preferably adapted to interact with the selected parts of the electronics in the car so as to make it possible to e.g. mute a stereo in the car when a call is received.
Regardless of the specific design of the hands-free unit, it may be operated by means of the voice rather than by means of the hands of the user, wherein the user provides predetermined commands orally to the mobile terminal for instructing the mobile terminal to perform specific tasks. Exemplary commands for use with such a device may be “answer”, “hang-up”, “call home” etc.
Even though the hands-free facilitates the use of the mobile terminal station while e.g. traveling by car, the environment in which the mobile station is used may not be well suited for oral communication. For example, the noise level in a car varies according to various conditions, such as the vehicle's acceleration and deceleration, entry to and exits from tunnels, the positions of the windows in the car, the character of the road on which the car is traveling, weather conditions, passengers riding in the car, etc. Noise levels exceeding 80 dB inside a car when using the car under normal conditions are common in many cars today. Besides the increased noise level inside the car when the car is moving, the small compartment of the car gives rise to other undesirable acoustic effects, such as echoes and frequency distortion, impairing the intelligibility of a telephone call made from inside the car.
It is known in the art to reduce the impact of echoes generated inside the car by providing suitable electronic equipment which electronically cancels the echoes by means of filtering the signal received by the microphone in the hands-free unit. More specifically, the echo-canceling equipment is usually in form of a digital signal processor (DSP) and an associated echo-canceling software which is executed in the DSP for reducing the influence of the echoes in the signal from the microphone. The algorithm used for modeling the echoes arising from the small compartment inside the car requires the calculation of a large amount of parameter values which are unique for each compartment.
As can be understood, the accuracy of the calculated parameter values are crucial for the overall performance of the echo canceling equipment, i.e. the conditions under which the audio data, serving as a base for the calculation of the parameter values, are recorded will affect the final values of the parameters in the echo-canceling algorithm. In case the parameter values are based on a recording made while the car is moving, the sources of interference mentioned above (the vehicle's acceleration and deceleration etc) will deteriorate the end result of the parameter value calculation. Moreover, if the compartment is empty apart from the driver of the car, the echo characteristics will be different than if one or more passengers are present inside the car. It is hence of outmost importance that the conditions under which audio data is recorded for the calculation of the parameter values are adapted to the actual driving conditions and that the audio recording in a true manner reflects the design of the interior of the car.
As to the frequency distortion arising from the environment in which the mobile terminal is used, e.g. when using the mobile terminal together with a hands-free unit in a car, the discussion above applies as well. The DSP in the mobile terminal may then perform a frequency correcting algorithm (frequency equalization), wherein frequencies being attenuated by e.g. the car upholstery are amplified in order to provide a higher-quality sound reproduction. Also in this case, the algorithm used for compensating the poor frequency response of the small compartment of the car requires the calculation of filter parameter values which are unique for each compartment.
U.S. Pat. No. 6,097,943 discloses an accessory item for performing echo-canceling in a mobile radio subscriber station. The accessory item includes a separate memory within which parameter values calculated by the processor of the mobile station and related to performance of the echo-canceling are stored. The parameter values are retrieved by the same or a different mobile station on a subsequent occasion and used to perform the echo-canceling without having to recalculate the parameter values. The parameter values are calculated and stored in the memory of the accessory item on the first occasion when the accessory item is attached to the mobile subscriber station. As can be understood from the above, the invention according to U.S. Pat. No. 6,097,943 put great demands on both the user and the conditions under which the recording is made when he is performing the initial parameter value calculation. Firstly, the user must be provided with specific information regarding the operation of the accessory device, i.e. how the echo-canceling functionality is achieved, in order for being able to provide the correct acoustic conditions for the initial recording. Secondly, the user must take into consideration how to establish the correct acoustic conditions in the car, i.e. shall the car be moving or not, shall there be any passengers present in the compartment, shall the car be in a garage, etc.
Furthermore, the mobile subscriber station to which the accessory item is attached must be able to provide the necessary computational power in order to provide a quick and correct calculation of the parameter values. Besides providing the parameter values to the accessory unit for subsequent use, the DSP in the mobile subscriber station must also be able to process the audio data received from the accessory unit in order to provide the desired echo-canceling or frequency correcting functionality described above. Such data processing put even greater demands on the DSP in the mobile subscriber station as to the provision of computational power, wherein it may be more efficient under certain circumstances to shift the computational burden over to the accessory device. Moreover, as discussed above the environment in which the mobile terminal is used, e.g. a small compartment inside a car, requires the calculation of a large amount of parameter values which are unique for each compartment. Even though the mobile terminal may calculate the required parameter values and use them in one or more general echo-canceling routines, the widespread use of mobile terminals in many different environments, which are not known at the time of manufacture of the mobile terminals, implies that the routines used for providing the audio processing functionality are not optimized for one specific environment in which the mobile terminal may reside.
Finally, since increased computational power of the DSP in the mobile terminal gives rise to increased energy consumption in total and hence a short battery operating time, it is of outmost importance that the computational power of the DSP in the mobile terminal is kept as low as possible still providing the necessary audio, video or image processing functionality.